Exploring the role of Mn2+ in the structure, magnetic properties, and radar absorption performance of MnxFe3−xO4–DEA/MWCNT nanocomposites

Iron oxide/carbon-based nanocomposites are known as an ideal combination of magnetic–conductive materials that were recently developed in radar absorption application; one example is the Fe3O4/multiwalled carbon nanotubes (MWCNTs). In this study, we try to boost their radar absorption ability by Mn-ion doping. Mn is an appropriate Fe substitute that is predicted to alter the magnetic properties and enhance the conductivity, which are crucial to developing their radar absorption properties. Diethylamine (DEA) is also used as a capping agent to improve the size and shape of the nanocomposite. In this study, a MnxFe3−xO4–DEA/MWCNT nanocomposite is successfully prepared by the coprecipitation method using a variation of x = 0, 0.25, 0.5, 0.75, and 1. We found that the sample's magnetic saturation (Ms) decreases, while the reflection loss (RL) increases with increasing the molar fraction of Mn. The enhancement of the radar wave absorption in the sample is dominated by dielectric losses due to the increase of electrical conductivity and interfacial polarization with the addition of Mn in the nanocomposites. We believe that our finding could shed light on the role of doping elements to develop the radar absorption properties, and further pave the way for the real implementation of iron oxides/graphene-based nanocomposite as radar-absorbing materials (RAMs).


Introduction
Recently, the escalating growth of communication technology, particularly wireless communication, has led to a signicant rise in electromagnetic pollution, emerging as a critical concern. 1 Electromagnetic pollution damages electronic devices, and has a detrimental effect on human health. 2,3herefore, researchers are currently working on the development of microwave-absorbing materials, particularly radarabsorbing materials (RAMs).Apart from addressing the issue of electromagnetic pollution, RAMs have potential applications in the military sector to conceal military defense equipment, such as ghter planes and tanks, from adversaries. 4RAMs attenuate the energy of electromagnetic waves by converting the waves into heat energy through magnetic and dielectric losses.Nevertheless, the primary challenge in RAM design lies in the selection of materials.To maximize their capability to absorb electromagnetic waves, a better understanding of how to engineer materials composed of magnetic and dielectric elements is required.
Fe 3 O 4 nanoparticles (NPs) have garnered attention as potential microwave absorbents 5,6 owing to their unique features, including outstanding magnetic properties, 7,8 great permeability, excellent chemical and thermal stability, 9 high biocompatibility, and nontoxicity. 10Despite these superiorities, Fe 3 O 4 still has relatively low absorption 8 and decient dielectric properties.3][14] However, they fail to generate NPs with uniform size and shape.Diethylamine (DEA) is one of the capping agents serving as a so template for improving the size and shape of NPs to minimize agglomeration, which is also important to boost the absorption ability. 15,16he performance of Fe 3 O 4 NPs in radar absorption can be improved through compositing and substitution with other materials.Recently, carbon nanotubes (CNTs) have gained attention as composite materials with Fe 3 O 4 NPs owing to their a Department of Physics, Faculty of Mathematics and Natural Science, State University of Malang, Jl.Semarang 5, Malang, 65145, Indonesia.E-mail: ahmad.tauq.fmipa@um.ac.id dielectric, mechanical, electrical, and thermal properties and extensive surface area. 17Multiwalled CNTs (MWCNTs) are one of the CNTs commonly composited with Fe 3 O 4 NPs for numerous applications, including RAMs. 18A combination of carbon and magnetic materials is considered as an effective strategy to obtain high-performance RAMs. 19For RAM application, MWCNT offers excellent conductivity. 20Combining MWCNT with Fe 3 O 4 enhances the absorbing intensity and bandwidth and reduces RAM density, which benets the performance of the RAM. 21Fe 3 O 4 as a magnetic component also could enrich the electromagnetic wave dissipation mechanism, which is important in wave absorption ability. 22Nevertheless, although the iron oxides/graphene-based nanocomposite is known as an ideal combination of magnetic-conducting materials that was recently developed in radar absorption applications, further development is required.Various attempts had been carried out, such as element/ion substitution, shape or morphology modication, and compositing strategies.Among them, we believe that element/ion substitution could substantially develop the RAM.Thus, an in-depth exploration of this strategy is very important.
In this study, we performed Mn 2+ ion substitution to improve the radar absorption ability and performance of Fe 3 O 4 -DEA/MWCNT.Mn 2+ ion substitution can alter the magnetic properties of iron oxides since it has the appropriate atomic radii with Fe. 23 Magnetic properties are one of the vital parameters for Fe 3 O 4 NP application in radar absorption application. 24Mn ion substitution can also increase the conductivity of a material, including Fe 3 O 4 , which affects the dielectric loss of the RAM. 23,24This study aims to identify the effects of Mn 2+ ion substitution on the structure, morphology, and magnetic properties of Fe 3 O 4 -DEA/MWCNT nanocomposites.An in-depth study of the effect of Mn substitution is very important to pave the way for the real implementation of Fe 3 O 4 -DEA/MWCNT and other types of iron oxides/ graphene-based nanocomposites as RAMs.

Experimental method
2.1.Synthesis of Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites Iron sand was used as the central precursor in the Mn x Fe 3−x O 4 preparation.MnCl 2 $6H 2 O, DEA, HCl (12 M, 99.9%), NH 4 OH (6.5 M, 999%), and HNO 3 (37%) were purchased from Merck.MWCNTs were obtained from Sigma-Aldrich, and distilled water was acquired from Pro Analysis.MWCNTs were functionalized as previously described. 25First, 1 g of MWCNTs was mixed with 100 mL of HNO 3 , and sonicated for 2 h at 40 kHz and 50 °C.The solution was ltered, washed using distilled water until pH 7, and then dried in an oven at 100 °C for 5 h to produce functionalized MWCNT powder (F-MWCNT).The synthesis of nanocomposites began with the production of Mn x Fe 3−x O 4 .First, iron sand was separated using a permanent magnet to select the magnetic powder with high purity. 26Then, 20 g of magnetic powder was reacted with 58 mL of HCl, followed by stirring for 30 min at room temperature to produce FeCl 2 and FeCl 3 solutions.MnCl 2 $6H 2 O with Mn x fraction variations of x = 0, 0.25, 0.5, 0.75, 1 was then added.We controlled the amount of molar fraction x with the calculated Mol divided with Mr, and reaction of Mn x Fe 3−x O 4 followed eqn (1).
The solution was then mixed for 20 min, and titrated with 6 mL of DEA solution previously dissolved into 9 mL of distilled water.The titration products were combined with 0.1 g of F-MWCNT while stirring at room temperature.A total of 19 mL of NH 4 OH was titrated until a black precipitate was obtained.The precipitate was washed using distilled water until pH 7, and then dried at 100 °C to attain a sample of the Mn x Fe 3−x O 4 -DEA/ MWCNT powder.

Characterizations
The structure, phase, crystallite size, and lattice parameter of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites were investigated by X-ray diffraction (XRD) using X'Pert Pro test, Cu-Ka 1.540 Å Panalytical Merck with Cu-Ka 1.54060 Å radiation beam.The morphology and components of the nanocomposites were determined by scanning electron microscopy (SEM) combination of carb characterization type FEI, Inspect-S50.The functional groups within the sample were investigated using Fourier transform infrared (FTIR) spectroscopy type IRPrestige-21.The magnetic properties of the nanocomposites were characterized using a vibrating sample magnetometer (VSM) type PPMS Ver-saLab with a magnetic eld ranging from −3 to 3 Tesla.Lastly, the radar absorption performance of the nanocomposites was characterized using vector network analyzer (VNA) type Rohde-Schwarz ZVA 67 to determine the complex permittivity, complex permeability, and reection loss (RL).

Results and discussion
The XRD pattern of Mn x Fe 3−x O 4 -DEA/MWCNT is presented in Fig. 1.Diffraction peaks with x = 0 code (Fig. 1) were detected at 2q = 30.21°,35.57°, 43.13°, 53.65°, 57.13°, and 62.69°.This diffraction pattern is similar to that reported for Fe 3 O 4 -DEA. 15he absence of new peaks following DEA addition suggests the successful utilization of DEA as a surfactant. 16Furthermore, the lack of MWCNT diffraction peaks at 2q = 26°is caused by the lower mass of MWCNT relative to that of Mn x Fe 3−x O 4 at a ratio of 1 : 30 for MWCNT : Mn x Fe 3−x O 4 .The diffraction patterns of all samples also indicate that the addition of Mn as a dopant (x = 0.25-1) generates the same pattern as that of the x = 0 sample.Therefore, our results showed that Mn addition causes no new peak or phase, signifying that Mn has successfully penetrated Fe 3 O 4 .Li et al. demonstrated that the absence of new peaks on Mn x Fe 3−x O 4 indicates that Mn 2+ successfully enters Mn x Fe 3−x O 4 , replaces Fe 3+ , and does not form MnO 2 deposits on the Fe 3 O 4 surface. 27e performed a quantitative analysis by comparing the obtained data with the ICSD database number 30860.The results showed that the observed peaks are identical with the Miller index of ( 220), (311), (400), ( 422), (511), and (440), with the highest peak observed at ∼2q = 35.5°on the hkl (311).This nding indicates the single phase of the sample with cubic spinel structure and the space group of Fd3m.The diffraction patterns at hkl (220) and (311) demonstrate a shi towards smaller 2q values with Mn substitution.This shi is associated with the increase in the sample's lattice parameters (8.361-8.429Å) due to Mn substitution, as summarized in Table 1.The observed increase in lattice parameter is attributed to the effects of ionic size because Mn 2+ (0.81 Å) possesses a larger ionic radius compared to Fe 3+ (0.77 Å). 28 Therefore, Mn substitution on Fe 3 O 4 drives the expansion of cell units, as proven by the expanded sample's crystal volume shown in Table 1 and Fig. 2.This nding also signies that some Fe 3+ ions on the tetrahedral structure have been substituted by Mn 2+ ions. 24Our results are similar to those of a previous study reporting an increase in lattice parameters from 8.372 Å to 8.474 Å with Mn 2+ substitution on x = 0.25 to x = 1 variations.The crystal structure of Mn x Fe 3−x O 4 is illustrated in Fig. 2. The x = 0 sample structure has an inverse cubic spinel structure with eight Fe 3+ ions in the tetrahedral site, and eight Fe 2+ and Fe 3+ ions in the octahedral site.The addition of Mn 2+ has enlarged the cubic spinel crystal structure compared to that of the sample x = 0, signifying the expansion of the cell unit.In samples with x = 0.25, 0.5, and 0.75, Mn 2+ ions replace some Fe 3+ ions on the tetrahedral site.In sample x = 1, all Fe 3+ ions are substituted by Mn 2+ ions.Four and six O ions are observed surrounding the Fe 2+ , Fe 3+ , and Mn 2+ ions on the tetrahedral and octahedral sites, respectively.Fig. 3 displays the graph depicting variations in the lattice parameters and Mn molar fraction variations.The Mn molar fraction demonstrates an increasing trend with two distinct slopes: one for molar fractions ranging from 0 to 0.5, and another for molar fractions from 0.5 to 1.This observed trend closely resembles the increase in lattice parameters reported in a previous study of about ∼8.4 Å. 30 Additionally, this trend is consistent with the Mössbauer results, which indicate that Mn 2+ ions replace Fe 3+ ions on the tetrahedral sites.This nding is supported by the Mössbauer spectroscopy, which shows that the iron ions on the MnFe 2 O 4 system are in the 3+ state, allowing the substitution of Mn 2+ to be written as The crystallite size of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites decreases with the increase in Mn substitution, which is consistent with previous results. 23The reduced crystallite size is caused by the addition of DEA as the controlling and reducing agent for the crystallite size.Tauq et al. similarly reported that all synthesized Fe 3 O 4 samples with the DEA template present a nanometric particle size, which decreases upon the addition of DEA at high concentrations. 16By using DEA, the particle size of the nanocomposite is easy to control.Thus, by controlling the particle size, we also can tune the absorbance ability of the nanocomposite. 19he morphology of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites was assessed by SEM characterization.The SEM  The carboxyl group with a negative charge will interact with Mn x Fe 3−x O 4 having positive ions (Mn 2+ , Fe 2+ , and Fe 3+ ), which initiate an electrostatic attraction and increase the structural stability of the nanocomposites. 31,32Due to this fact, Mn x -Fe 3−x O 4 is strongly bonded to the surface of MWCNT.MWCNT also serves as a barrier to suppress the aggregation tendency of Mn x Fe 3−x O 4 , and thus further increases the structural stability. 33In addition, the morphology of MWCNTs that form a long tube shape tends to have better conductivity because they can form a more continuous conductive path.This agrees with previous reports indicating that RAMs with tubular shape and considerable anisotropy can form three-dimensional conduction networks to enhance its conductivity. 34This condition favors wave propagation through the material and its attenuation.Thus, an excellent wave attenuation in this proposed RAM could be induced due to the excellent interface between CNTs and Mn x Fe 3−x O 4 , which creates an interface polarization. 35It is important to note that the interface polarization could induce excellent dielectric loss capacity for microwave absorption of the nanocomposite. 34Controlling the shapes and morphology of RAMs is important because the microwave absorption ability also depends on the degree of density, weight and dispersion of RAMs itself. 36The morphology of the nanocomposites aer Mn addition, as depicted in Fig. 4, closely resembles the nanocomposites with a molar fraction of 0.Moreover, we observed a decrease in agglomeration with Mn addition.On the other hand, the further reduction of agglomeration upon inclusion of DEA was also indicated. 18,37ig. 6 shows the FTIR spectrum of the Mn x Fe 3−x O 4 nanocomposites.At 3491-3290 cm −1 , we identied a widened O-H vibration, possibly induced by the -OH groups on the surface areas.This functional group facilitates the combination of Mn-Fe 2 O 4 with MWCNT. 15,38Our obtained vibrations are similar to the vibrational peak of O-H reported in previous studies, specically those at 3321, 25 3420, 38 and 3430 cm −1 . 15e also detected the C-O functional groups at 2325 and   1359 cm −1 that are possibly from the CO 2 atmosphere 15 and identied the C]C functional group at 1438 cm −1 , characterizing the graphite bonds compiled in the MWCNT.No functional groups of DEA are found, especially N-H, C-N, and N-H at 733, 1143, and 3288 cm −1 , respectively.This nding suggests that the DEA used in nanoparticle preparation has not evaporated.The functional groups of metal oxide (M-O), namely, Fe-O and Mn-O, are detected at 412 and 673 cm −1 .These two vibrations indicate that the manganese ferrite has a cubic spinel structure at the octahedral and tetrahedral sites.This nding is in line with a previous study reporting vibration peaks in the range of 430-482 cm −1 and 433-573 cm −1 . 39As illustrated in Fig. 6, the absorbance intensity between 600 and 700 cm −1 increases with further addition of Mn molar fraction.This nding conrms the success of the Mn 2+ ions in substituting Fe 3+ ions in the tetrahedral sites.In addition, the increasing concentration of Mn leads to a shi of absorbance peaks towards lower wavenumbers.This phenomenon is correlated with the increase in the distance between two atoms, which occurs due to the increase in lattice parameters.
Fig. 7 shows the M-H hysteresis curve from the VSM analysis of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites.All samples show superparamagnetic features as indicated by the obtained S curve.The attained hysteresis curve was then analyzed using the Langevin method with susceptibility, 40 and the results are summarized in Table 2.The remanent magnetization and coercivity eld are close to 0, characterizing the superparamagnetic  Paper RSC Advances components. 30The highest magnetization of 32.32 ± 0.03 emu g −1 is observed for Mn x Fe 3−x O 4 -DEA/MWCNT at x = 0.This value is lower than the previously reported magnetization of Fe 3 O 4 -DEA at 35.76 emu g −1 .Therefore, the obtained values can be correlated with the addition of MWCNT (nonmagnetic), i.e., MWCNT lowers the magnetic fraction of the samples and affects the M s value.In our previous study, we uncovered that Fe 3 O 4 / MWCNT has M s of 12.64 and 21.15 emu g −1 , which are lower than the magnetization values obtained in the present work.The addition of DEA to the nanocomposites also affects the sample's magnetization.The magnetization values also decrease following the increase in the Mn molar fraction substituted for Fe 3 O 4 .This decrease can also be attributed to the low crystallite value, as demonstrated in Table 1.Nanosized particles typically have a single domain and exhibit paramagnetic behavior, resulting in an increased spin disorder on the nanoparticle's surface, which subsequently reduces the magnetic moment. 41he real and imaginary parameters of the electrical permittivity (real permittivity (3 ′ ) and imaginary permittivity (3 ′′ )) and the magnetic permeability (real permeability (m ′ ) and imaginary permeability (m ′′ )) shown in Fig. 8 are parameters used to determine the absorption mechanism of the Mn x Fe 3−x O 4 -DEA/ MWCNT nanocomposites for radar waves.3 ′ and m ′ represent the ability to store electric and magnetic energy, respectively; and 3 ′′ and m ′′ are related to the dissipation and loss of electric and magnetic energy, respectively.As illustrated in Fig. 8(a), which is the 3 ′ curve, the real permittivity value decreases from 6.4 to 4.2 when the molar fraction of Mn increases.Meanwhile, the 3 ′′ value tends to be greater aer Mn addition, which can be explained by eqn (2).
According to eqn (2), an increase in 3 ′′ is related to electrical conductivity.Mn substitution increases the conductivity value of a material due to the production of holes during charge transfer. 42An increase in electrical conductivity also increases the loss and dissipation ability of the electric part, as indicated by the 3 ′′ value of the nanocomposites with x = 1 greater than x = 0.In addition to electrical conductivity, the 3 ′′ value is related to interfacial polarization, which is marked by several resonance peaks around the frequencies of 7.5, 8.5, 9.5, 11, and 12.5 GHz.These peaks are related to the relaxation stage when interfacial polarization occurs. 2 The m ′ and m ′′ of Mn x Fe 3−x O 4 -DEA/MWCNT are shown in Fig. 8(c) and (d), respectively.The m ′ of all samples tends to decrease gradually, and the m ′′ increases with the frequency.Fluctuations are observed in the m ′ and m ′′ curves, especially at frequencies above 9 GHz.The uctuation peaks represent the resonance peaks, where the absorbance peaks at low frequencies originate from the wall resonance domain, and the peaks at high frequencies correspond to natural resonance.Furthermore, the m ′′ of the sample with Mn addition is greater than that of the sample without Mn substitution.This nding indicates that the magnetic loss of the nanocomposites increases with the molar fraction of Mn.The m ′′ in the sample x = 0 shows a negative value at several frequencies, implying that the magnetic energy from this sample is directly emitted without being absorbed at that frequency. 43n In general, the absorption capability of radar waves is described by magnetic loss and dielectric loss, which are represented by tan d m and tan d 3 , respectively.tan d m and tan d 3 describe the loss capacity of the radar absorption materials, and can be calculated using eqn (3) and (4), respectively.Fig. 9 shows the relationship of tan d m and tan d 3 with frequency.Comparison between Fig. 9(a) and (b) shows that the tan d 3 value is higher than the tan d m value.These results explain that dielectric loss is the main contributor to the absorption of radar waves by the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites. 44he performance of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites for the absorption of radar waves is shown as a graph of the relationship between the reection loss (RL) and frequency depicted in Fig. 9(c).The RL values of Mn x Fe 3−x O 4 -DEA/MWCNT for x = 0-1 are −4.39,−4.85, −5.77, −6.26, and −7.03 dB.The increase in RL is associated with a decrease in particle size as the molar fraction of Mn increases in the nanocomposites.Nanocomposites with a small size have a large proportion of surface atoms, which can be magnetized and polarized under an external electromagnetic eld.This situation allows the radar wave energy to be converted into heat, resulting in high RL values. 45The highest RL of −7.03 dB is recorded for the sample with x = 1, which is consistent with the highest tan d m and tan d 3 values (magnetic and dielectric loss) for this sample.In addition, x = 1 has a smaller crystallite size than other samples (Table 1).A low particle size is associated with a high dipole polarization that induces dielectric loss. 46In addition, the particle sizes also affect the degree of density, weight, and dispersion of nanocomposites, which further affect their absorption ability. 36

Conclusion
Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites have been successfully synthesized by the coprecipitation method having Mn fraction variations of x = 0, 0.25, 0.5, 0.75, 1.The crystallite size decreases from 17 to 4.5 nm with the increase in substituted Mn substitution.The morphology of the nanocomposites contains spherical shapes and chunks of Mn x -Fe 3−x O 4 , as well as tubular shapes of the MWCNT, indicating they are physically contacted.All of the synthesized nanocomposites exhibit superparamagnetism with the tendency of decreasing saturation magnetization at around 32.32 ± 0.03 to 11.51 ± 0.02 emu g −1 due to Mn addition, which presumably enhances the spin disorder on the nanocomposite's surface.Furthermore, the performance of the nanocomposites in absorbing radar waves is shown by the RL value that increases from −4.39 to −7.03 dB with the increase in the molar fraction of Mn.It shows that the radar absorption performance of the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites is dominated by dielectric loss, owing to increased electrical conductivity and interfacial polarization with the addition of Mn.We believe that our ndings may shed light on the role of substitution elements in developing the radar absorption properties not only for Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites, but also for other iron oxides/graphene-based nanocomposites, which further pave the way for their real implementation as RAMs.

Fig. 1
Fig. 1 Diffraction pattern of Mn x Fe 3−x O 4 -DEA/MWCNT with different compositions of Mn substitution.

Fig. 3
Fig. 3 Relationship of the Mn molar fraction (x) with the lattice parameter and crystal volume.

Fig. 6
Fig. 6 FTIR spectrum for the Mn x Fe 3−x O 4 -DEA/MWCNT nanocomposites with different compositions of Mn substitution.
Fig. 8 (a)-(c) Real and (b)-(d) imaginary parts of the permittivity and permeability of Mn x Fe 3−x O 4 -DEA/MWCNT with different compositions of Mn substitution, respectively.

Fig. 9
Fig. 9 (a) tan d m , (b) tan d 3 , and (c) RL of Mn x Fe 3−x O 4 -DEA/MWCNT with different compositions of Mn substitution.

Table 2
Magnetization saturation, remanent magnetization, coercivity field, and susceptibility of the Mn x